Traversal is largely unchanged from BSTs. However, we can be confident
that the tree won't easily exceed the maximum stack height, because of
the AVL balance condition, so we can omit checking for stack overflow.

We do need to make a new implementation of the insertion traverser
initializer. Because insertion into an AVL tree is so complicated, we
just write this as a wrapper to avl_probe(). There probably wouldn't
be much of a speed improvement by inlining the code anyhow: